Chemistry Reference
In-Depth Information
most important factor and should be carefully controlled at about 7.0, because the
monolacunary [BW
11
O
39
]
9-
is only stable at a narrow pH range (pH 7-8). When the pH is
lower than 6, it transforms into [BW
13
O
46
H
3
]
8-
at room temperature or the saturated
[
D
-BW
12
O
40
]
5-
while increasing the temperature. When the pH is larger than 8, the
polyanion may be destroyed only leaving borate and tungstate in solution.
[83]
There is
also evidence that [BW
11
O
39
]
9-
is not existing in solution due to its high negative charge
and is only stable as [BW
11
O
39
H]
8-
.
[84]
So the frail synthesis gives rise to expect a considerable amount of side products from
the reaction why the functionalized clusters were crystallized from the solution and not
directly phase transferred. For detailed synthetic instructions please see the
experimental section as well as the adopted procedures from literature.
[77, 85, 86]
The bor
templated POM-ILs could not be obtained in satisfying purity so that a comparison with
the other ILs synthesized in this work concerning their physical properties isn´t
reasonable at this point.
3.3. Variation of the cation - phosphonium and imidazolium POM-
ILs
Tetraalkylphosphonium polyoxometalate-based POM-ILs were first reported in 2007 as
a different approach towards true POM-ILs. Here, the choice of organic cation was
rather straight-forward as tetraalkylphosphonium salts of POMs are renowned for their
facile preparation as well as their high thermal stability.
[67, 87]
The authors screened a
range of imidazolium and phosphonium cations, several of which only led to high
melting-point (> 200 ˚C) solids which were not further investigated. However, the
combination of trihexyltetradecylphosphonium, (C
14
H
29
)(C
6
H
13
)
3
P
+
cations with Keggin
[PW
12
O
40
]
3-
and Lindqvist [W
6
O
19
]
2-
anions gave true POM-ILs with melting points of
65 ˚C (Keggin) and ˗48 ˚C (Lindqvist), respectively. Thermal stability measurements using
TGA-DSC analysis showed that no weight loss is observed up to
ca.
480 ˚C; long-term
heating experiments further showed that
POM-IL-2
(see. chapter 1.4 for details) stays
intact for prolonged periods at temperatures up to 360 ˚C, so that a drastically higher
thermal stability is observed for
POM-IL-2
compared with
POM-IL-1
.
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